1. Field of the Invention
The present invention relates to nanoporous materials, methods for the manufacture of nanoporous materials and applications of nanoporous materials. Such materials have utility particularly, but not exclusively, in separation technology, e.g. in separation membranes and the preparation of high surface area electrodes for energy generation and energy storage (e.g. battery and/or supercapacitor) technology.
2. Related Art
Block copolymers are a class of macromolecules comprising two or more chemically distinct polymer blocks. It is conventional to refer to each polymer block as A, B, C, etc.
Hillmyer (2005) [Marc A. Hillmyer “Nanoporous Materials from Block Copolymer Precursors” Adv Polym Sci (2005) 190: 137-181] sets out a review of the formation of nanoporous materials using block copolymer precursors. Such nanoporous materials are of interest for use as nanolithographic masks, separation membranes, interlayer dielectrics and nanomaterial templates. Many synthetic techniques are known for generating AB diblock, ABA triblock and ABC triblock copolymers.
In, for example, linear AB diblock copolymers, four equilibrium morphologies have been identified: lamellar, cylindrical, bicontinuous gyroid and spherical. The morphology formed depends on factors including the relative volume fraction (or number fraction of monomeric units) of each block, and how unfavourable it is for the distinct blocks to mix.
Hillmyer (2005) reviews work on the preparation of ordered block copolymer materials and the subsequent selective etching of the minority component in order to form nanoporous materials. For example, the formation of a cylindrical morphology, and the subsequent etching of the cylinder-forming phase, results in the formation of nanoscopic channels through a matrix material (the matrix material being the continuous phase which surrounded the cylinder-forming phase).
Hillmyer (2005) points out that since there is a wide range of block copolymer structures available, nanoporous materials with a wide range of tunable properties can be imagined. However, he also points out that there are two key requirements for preparing nanoporous materials from ordered block copolymers: (i) the etchable material must be physically accessible to the solvent, and (ii) the matrix material must be able to support the resultant nanoporous structure. (ii) is typically achieved by cross-linking the matrix material.
Various block copolymer systems are discussed by Hillmyer (2005), including:
PS-PBD: polystyrene-polybutadiene
PS-PI: polystyrene-polyisoprene
PS-PBD: polystyrene-polybutadiene
PS-PEO: polystyrene-poly(ethylene oxide)
PPS-PI-PPS: poly(4-vinylphenyl-dimethyl-2-propoxysilane)-b-polyisoprene-b-poly(4-vinylphenyl-dimethyl-2-propoxysilane)
PtBA-PCEMA: poly(t-butylacrylate)-b-poly(2-cinnamoylethyl methacrylate)
PS-PMMA: polystyrene-b-poly(methyl methacrylate)
PS-PLA: polystyrene-polylactide
PI-PLA: polyisoprene-polylactide
PCHE-PLA: polycyclohexylethylene-polylactide
PαMS-PHOST: poly(α-methyl styrene)-b-poly(4-hydroxy styrene)
PS-PFMA: polystyrene-b-poly(perfluorooctylethyl methacrylate)
PI-PCEMA-PtBA: polyisoprene-b-poly(2-cinnamoylethyl methacrylate)-b-poly(t-butylacrylate)
PS-PVP: polystyrene-b-poly-4-vinylpyridine
P(PMDSS)-PI-P(PMDSS): poly(pentamethyldisilylstyrene)-b-polyisoprene-b-poly(pentamethyldisilyIstyrene)
PS-PDMS: polystyrene-polydimethylsiloxane
Lee et al (1989) [Lee J-S, Hirao A, Nakahama S “Polymerization of monomers containing functional silyl groups. 7. Porous membranes with controlled microstructures” (1989) Macromolecules 22:2602] report the preparation of porous membranes formed from PPS-PI-PPS. Lamellar, cylindrical and spherical morphologies were obtained. When the PI was degraded and removed, the lamellar and cylindrical morphology membranes had a high degree of open porosity, as measured by adsorption of nitrogen (using the BET (Brunauer-Emmett-Teller) method). For the spherical morphology, although the PI could be degraded and removed, the pores are characterised by Lee et al (1989) as closed, because substantially no nitrogen adsorption was observed.
Hillmyer (2005) also refers to other work on spherical morphology block copolymers, this time in bulk form, in which the spherical phase was removed using an anhydrous HF etch. The material studied was PS-PDMS. The byproducts of the etching procedure are volatile and can be removed by evacuation, but the remaining spherical pores were closed.
It is also known to blend block copolymers with homopolymers, e.g. PS-PI with PS homopolymer. Hillmyer (2005) reviewed work on this material to form a bicontinuous gyroid phase. Such a morphology is of interest because orientation of the morphology is not required in order to form separation membranes.
Peinemann et al (2007) [K. V. Peinemann, V. Abetz, P. F. W. Simon “Asymmetric superstructure formed in a block copolymer via phase separation” Nature Materials Vol. 6 Dec. 2007. pp. 992-996] disclose work on the formation of isoporous membranes using PS-PVP diblock copolymer. PVP is a hydrophilic polymer that dissolves in dimethyl formamide (DMF), lower alcohols and aqueous mineralic acids. PS is a hydrophobic polymer that dissolves in a number of organic solvents such as toluene, tetrahydrofuran (THF) or chloroform. PS-PVP diblock copolymers are strongly segregated and their morphology is controlled by the block ratio. A hexagonally packed array of PCP cylinders can be expected for a volume fraction of PVP in the range 0.12 to 0.31. Peinemann et al (2007) therefore studied PS-PVP with 15 wt % PVP.
Crossland et al (2009) [Edward J. W. Crossland, Marleen Kamperman, Mihaela Nedelcu, Caterina Ducati, Ulrich Wiesner, Detlef-M. Smilgies, Gilman E. S. Toombes, Marc A. Hillmyer, Sabine Ludwigs, Ullrich Steiner, and Henry J. Snaith “A Bicontinuous Double Gyroid Hybrid Solar Cell” Nano Lett., Vol. 9, No. 8, 2009, pp. 2807-2812] disclose the use of a porous structure formed from double gyroid phase morphology diblock copolymer to form a solar cell. TiO2 was deposited in the pores. The remaining matrix material from the diblock copolymer is then removed to leave a freestanding network of TiO2. An organic semiconductor material is then infiltrated into the pores in the TiO2 network to form a bicontinuous heterojunction solar cell architecture.
Li et al (2010) [Xianfeng Li, Charles-Andre Fustin, Nathalie Lefevre, Jean-Francois Gohy, Steven De Feyter, Jeremie De Baerdemaeker, Werner Eggere and Ivo F. J. Vankelecom “Ordered nanoporous membranes based on diblock copolymers with high chemical stability and tunable separation properties” J. Mater. Chem., 2010, 20, 4333-4339] note that it is known to form porous membranes by selective removal of the oriented cylindrical phase from diblock copolymer films. However, their work addresses the problem of subsequently transferring the porous membrane to a porous support for practical use of the membrane in separation techniques. PS-PEO diblock copolymer with added PM (poly(acrylic acid)) is deposited onto a porous ceramic (alumina) support by spin coating, forming a layer over the ceramic support. By careful control of the composition, a cylindrical array of PEO-PAA is formed in the PS matrix. Cross-linking of the diblock copolymer is achieved by UV exposure.
U.S. Pat. No. 5,948,470 discloses the formation of a thin film of PS-PI diblock copolymer in which PI spheres are formed in a PS matrix. The packing of the spheres (at least in bulk samples) is body centred cubic. The PI spheres are removed by ozonolysis and the resultant film used for nanolithography.